A master oscillator followed by one or more power amplifiers, the so-called MOPA structure, is frequently used in lasers that generate output powers with hundreds or thousands of watts. The output could be in the form of continuous amplitude without modulation in time, the so-called CW output, or of regulated Q-switched pulses.
In a MOPA laser, the master oscillator is used to generate an output that meets all requirements of the desired application. These requirements typically include low noise, a round beam with a good spatial mode profile without high-frequency structure, a low beam divergence (M2) limit and/or a pulse width limit in Q-switched operation. The oscillator output power is, in most cases, low to medium, so that it can accept less robust optical components and advanced designs to achieve the above requirements. The oscillator output is sent to the power amplifier where the power level is magnified to the required level while maintaining the main characteristics of the oscillator output.
Mode matching optics are used between the oscillator and the amplifier to transfer the oscillator output beam to the amplifier laser rod(s) with a matched beam size so that most of the gain in the amplifier is utilized. In addition, the laser rod size of the amplifier has to be carefully chosen so that the power or energy density of the oscillator beam is higher than the saturation density to achieve high extraction efficiency. Other subtle considerations on the amplifier rod size selection include avoiding spatial mode as well as temporal profile distortion due to gain saturation.
Previous mode matching optics consist of either a single or multiple lenses or curved mirrors and have been demonstrated mostly in flashed lamp pumped Q-switched MOPA systems where repetition rates are in the order of 1-50 Hz (See, Norman Hodgson and Horst Weber, Laser Resonators and Beam Propagation, 2nd ed., chapter 13.1 and 13.2, Springer 2005). In these low repetition-rate systems, each laser pulse is generated within each pump flash that lasts a few hundred microseconds. Within such a flash, thermal loading to the laser rod is not fast enough to spread out and establish an equilibrium thermal lens profile. Without sensing thermal lensing, the propagation parameters of the laser beam change very little as the pump power or repetition rate is readjusted. As a result, the mode matching optics optimized for a particular operating condition could be extended to the entire system operation.
In diode pumped or CW lamp pumped systems, the thermal lens created in the laser rods can no longer be ignored. In addition, as the pump power or repetition rate is adjusted during operation, the change in the focal length of the thermal lens can easily be on the order of tens of centimeters, thus creating a large change to the propagation parameters of the oscillator beam. In this case, the above-mentioned mode matching optics will no longer work. It is our desire to come up with a design for mode matching optics that addresses the presence of the thermal lenses in the rods. The mode matching optics should work to let the MOPA generate an output with high amplifier efficiency within a large range of pump power and repetition rate variations. It is also our desire that the optics function over a wide pump power operation range such that there is no need to readjust the mode matching optics to fit the different thermal lens values.
As noted above, in a CW diode or lamp pumped solid-state laser, the thermal lens heavily affects the propagation parameters of the output beam. These parameters include the beam size and the position of the waist if the output coupler is not flat. In addition, in most industrial applications, the pump power or repetition rate is frequently adjusted, for example, to suspend lasing operation between targets and to allow the laser beam to treat different work targets with different power density. In these cases, large thermal lens variations are induced. In a prior art MOPA system, wherein the output of the oscillator is sent to the amplifier by mode matching delivery optics, the change of propagation parameters due the changing thermal lens necessarily degrades the amplifier efficiency if the mode matching optics as either designed and optimized without the consideration of the thermal lens or optimized at only one value of pump power or repetition rate.
As noted above, a prior art system that uses a relay lens pair to image the oscillator rod(s) to the amplifier rod(s), will not work well in the presence of thermal lens. One prior approach used to address the problem included using an amplifier gain rod having the same diameter as the gain rod in the master oscillator. In the latter method, all amplifier optics and positions are rendered as images of the oscillator and the whole MOPA is viewed as two oscillators periodically repeated each other in space. Although this method can address the problem of pump power/thermal lens changes, the requirement of using a same rod diameter in the amplifier necessarily limits critical design criteria of the amplifier such as input beam power density and amplifier gain, compromising the MOPA efficiency.
It would be desirable to create a fixed optical system that would allow the MOPA to efficiently operate over a large range of pump powers. Further, it would be desirable if that system could be configured so that the laser can operate effectively even if the diameters of the rods in the oscillator and amplifier were different.